Assuming that your son purchased the home with the other individual as "tenants in common," he is entitled to a one-half, undivided interest in the property, with the equal right to access and possess the whole property.
Your son's right to possess the whole property also gives him the right to rent or sell his one-half interest in the property. Practically speaking, however, it would be very difficult to rent or sell his one-half interest when the other owner is unwilling to sell his interest or move out.
Another option should be for your son simply to move out. However, if your son is a co-signer on a mortgage encumbering the property, he would still remain jointly responsible for the mortgage payments. Also, even if he was no longer an occupant of the property, he would remain legally liable for accidents occurring at the property (or damage to the property), and should take steps to insure the property and his interests in and against such risks.
As an owner in tenancy in common, your son also has the legal right to petition the court for an "action for partition." An action for partition is a judicial proceeding forcing the division of real property held jointly or as tenants in common. This proceeding would essentially force a sale of the property allowing your son to recover the value of his undivided one-half interest, whether or not the other owner was willing to purchase your son's share.
-- Michael T. Sawyier
